1. Field of the Invention
The invention relates to drill bits for gravity-drop cable drilling.
2. Description of the Prior Art
The technique of cable drilling uses a large relatively heavy drill bit that is suspended from a cable drill rig and dropped a distance of twenty-four to thirty-six inches into the bottom of the hole being drilled. The drill bit is dropped at a rate of thirty to sixty times per minute and a spin is imparted to the drill bit as it is dropped to cut a hole of circular cross section with a drill bit of less than circular cross section. Drill bits can be made in an integral construction with hardened cutting edges or blades, but these become dull and worn, and the driller is faced with the repair or replacement of a rather bulky item. Therefore, it has become a practice to mount detachable cutting members on the body of the drill bit.
Cable drilling gets its name from the cable on which the drill bit is suspended from the boom of a cable drilling rig. This type of equipment is more economical for drilling a well hole of relatively large diameter and substantial depth than rotary percussion equipment that is typically used in mining and construction. Such rotary percussion equipment is mounted on over-the-road vehicles, off-the-road vehicles or platforms and uses a string of drill pipes which must reach from the drill rig to the bottom of the hole and which must be driven by motors for both vertical and rotating movement. To drill holes of large diameter would require drill pipes that would be quite large and heavy, whereas in cable tool drilling the drill pipes are replaced by a wire line.
As seen in Sanderson, U.S. Pat. No. 1,995,043 and Sanderson, U.S. Pat. No. 2,022,055 early detachable blades for percussion drill bits were formed as wedge-shapd members with a cutting edge. Two such bits were later employed in cable drilling in a staggered configuration as shown in one of my prior U.S. Pat. No. 3,545,554. A further development in rotary percussion drilling was the provision of pivotable steel plates with tungsten carbide inserts as seen in Stebley, U.S. Pat. No. 3,536,150. These inserts are much harder and more durable than the steel plates in which they are embedded and provide greater efficiency in cutting through hard earth formations. In Stebley, the rotary percussion drill bit is reciprocated a distance of only three to five inches at a rate of 500-800 impacts per minute. The bit is rotated at 20 revolutions per minute or no more than 9 degrees per impact.
Cable drills, on the other hand, are provided with a means for winding up the cable during the lifting stroke and for imparting a spinning action to the drill bit as it travels through the downstroke to the bottom of the hole. This provides shear stresses of a magnitude unique to cable drilling against elongated axial fasteners that are used to attach metal plates to the bottom of the drill bit. The primary cutting members in Stebley are made pivotable and do not employ axial fastening members. Fixed drill plates with tungsten carbide inserts have not been known in larger sizes for cable drilling because a workable configuration for use of such plates with the bits commonly used in cable drilling operations has not been known. These commonly used bits include two-winged spudding bits, four-winged star bits and hollow scow bits.
The scow bit is a specialized bit with a hollow barrel section in which water and cuttings that have been mixed into a slurry in the bottom of a hole can be evacuated. This is accomplished by the bailing action of a valve within the barrel section that is forced open on the downstroke and pulled closed on the upstroke to trap a portion of the slurry. Kita et al., U.S. Pat. No. 4,083,415 shows a mining bit with a central opening like a scow bit, however, the Kita drill bit is rotated in small increments while in contact with the work, which is a different type of drilling action than found in cable drilling. Also in Kita, the detachable portion of the bit is formed in the shape of a cup which fits over one end of the drill bit shank and is held in place by balls that are received in a groove around the outside of that shank. Kita does not address the problem of forces on axial fasteners for plates used in the cable drilling environment.